CoreFX Ref is providing a simple setup to let you compile an existing VisualStudio project against the CoreCLR/CoreFX target.
It provides the necessary MSBuild configuration files and the CoreFX reference assemblies which can be used in your VisualStudio project to compile against CoreCLR/CoreFX.
To use this, update your project to import configs\CoreCLR.CSharp.targets instead of the default config which is usually one of $(MSBuildToolsPath)\Microsoft.CSharp.targets or
In addition make sure to define the following properties TargetFrameworkRootPath and FrameworkPathOverride to point where you checked out CoreFX Ref.
For example, if you have checked out the code in C:\CoreFX, then you should add:
<TargetFrameworkRootPath>C:\CoreFX</TargetFrameworkRootPath>
<FrameworkPathOverride>$(TargetFrameworkRootPath)\CoreCLR\v5.0</FrameworkPathOverride>